The Government's use of the royal prerogative to trigger the process of taking Britain out of the European Union is not being done on a "whim" or "out of a clear blue sky", the UK's highest court has heard.

Attorney General Jeremy Wright told 11 Supreme Court justices at the start of one of the most important constitutional cases in British legal history that it was the "logical conclusion of a process in which Parliament has been fully and consciously involved".

The Government's top law officer said that process was one in which Parliament had "resolved to put a clear and decisive question about our nation's future to the British people, and in which Parliament expected the Government to act on the answer they gave".

He added: "None of this means, of course, that Parliament will not be closely involved in the process of the UK's withdrawal from the EU over the coming months and years."

Mr Wright was presenting argument on behalf of the Government urging the panel of justices to overturn a High Court ruling on November 3.

The hearing started on Monday and will continue today.

In a decision that infuriated Brexiteers, three judges said Theresa May lacked power to use the royal prerogative to trigger Article 50 of the Lisbon Treaty and start the two-year process of negotiating Brexit without the prior authority of Parliament.

Mr Wright told the justices in London that the case was of "great constitutional significance in which there is understandable and legitimate interest both inside and outside this courtroom".

He said the High Court had reached the "wrong" decision. It was for the Government to exercise prerogative powers in the conduct of the UK's affairs on the international plane.

The position of those who had brought the case and others had always been "that they have no interest in derailing Brexit but only in defending Parliament's role in the process".

Mr Wright continued: "But if this is all about standing up for Parliament, I say Parliament can stand up for itself.

"When it comes to leaving the European Union, Parliament has had full capacity and multiple opportunities to restrict the executive's ordinary ability to begin the Article 50 process and it has not chosen to do so."

He said: "However much they may wish it had, those who support parliamentary sovereignty should, we submit, respect this exercise of parliamentary sovereignty too."

Mr Wright submitted that "in the context of this case the imposition of a legislative precondition by the courts which Parliament did not choose to impose itself, cannot be supportive of parliamentary sovereignty but must be positively inconsistent with it".

He said: "In the delicate balance of our constitutional settlement this court should, we submit, resist the invitation to make such an imposition."

If the historic appeal is unsuccessful, and any potential further appeal to the European Court of Justice in Luxembourg also fails, the Government's plans for Brexit could be thrown into disarray.

But Mrs May has made it clear she still intends to give an Article 50 notification by the end of next March to start the leave negotiations with 27 other EU countries.

The Government argues that the panel of High Court judges erred over Article 50 and its use was legally justified by the June 23 referendum vote in favour of quitting the EU.

The High Court ruling was won by Gina Miller, 51, an investment fund manager and philanthropist who was selected to bring the lead case.

Her case is being supported by "concerned citizens" drawn from all walks of life, including London hairdresser Deir Dos Santos, 37, who helped start the legal battle over Brexit but, say his lawyers, has been forced underground after receiving "vile" hate mail.

The Scottish and Welsh governments and the Attorney General for Northern Ireland are all intervening in the Supreme Court case. A ruling will not be given until the new year.

James Eadie QC, also appearing for the Government, said Parliament had arranged for the June 23 referendum through the 2015 EU Referendum Act on the clear understanding that the Government would implement its outcome.

Nothing in law suggested that, following victory for the Leave campaign, the Government's decision to give Article 50 notice required further legislation. It was self-evident that withdrawal from the EU would affect the rights and obligations of individuals in a serious way, said Mr Eadie.

Parliament knew that would be the effect, yet left the Government's prerogative powers untouched.

If the intention of Parliament had been to make leaving or withdrawing subject to primary legislation "not merely could it have said so, but undoubtedly would have said so", argued Mr Eadie.

"Its silence is consistent and compelling."

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